Binding agents for electrochemically active materials and methods of forming the same

ABSTRACT

In some embodiments, an electrode can include a current collector, a composite material in electrical communication with the current collector, and at least one phase configured to adhere the composite material to the current collector. The current collector can include one or more layers of metal, and the composite material can include electrochemically active material. The at least one phase can include a compound of the metal and the electrochemically active material. In some embodiments, a composite material can include electrochemically active material. The composite material can also include at least one phase configured to bind electrochemically active particles of the electrochemically active material together. The at least one phase can include a compound of metal and the electrochemically active material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of U.S.application Ser. No. 15/995,002 filed May 31, 2018, which claims thebenefit of U.S. Provisional Application No. 62/596,077, filed Dec. 7,2017. The entirety of the above referenced applications are herebyincorporated by reference.

BACKGROUND Field of the Invention

The present disclosure relates to electrodes, electrochemical cells, andmethods of forming electrodes and electrochemical cells. In particular,the present disclosure relates to binding agents for electrochemicallyactive materials.

Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. For the cathode,separator and anode to be rolled, each sheet must be sufficientlydeformable or flexible to be rolled without failures, such as cracks,breaks, mechanical failures, etc. Typical electrodes includeelectro-chemically active material layers on electrically conductivemetals (e.g., aluminum and copper). For example, carbon can be depositedonto a current collector along with an inactive binder material. Carbonis often used because it has excellent electrochemical properties and isalso electrically conductive. Electrodes can be rolled or cut intopieces which are then layered into stacks. The stacks are of alternatingelectro-chemically active materials with the separator between them.

SUMMARY

Example electrodes are provided. The electrode can include a currentcollector, a composite material in electrical communication with thecurrent collector, and at least one phase between the composite materialand the current collector. The current collector can include one or morelayers of metal. The composite material can include from greater than 0%to about 99% by weight of electrochemically active material. The atleast one phase can be configured to adhere the composite material tothe current collector. In addition, the at least one phase can include acompound of the metal and the electrochemically active material.

In various electrodes, the current collector can comprise copper,nickel, iron, titanium, molybdenum, stainless steel, chromium, aluminum,or a combination thereof. The electrochemically active material cancomprise silicon, graphite, germanium, tin, silicon oxide (SiOx),aluminum, or a combination thereof. In some instances, the compound caninclude a metal silicide. For example, the metal silicide can comprisecopper silicide, nickel silicide, chromium silicide, aluminum silicide,titanium silicide, or a combination thereof.

In some electrodes, the current collector can have a surface roughnessfrom about 0.025 Rz/μm to about 25 Rz/μm. For example, the currentcollector can have a surface roughness from about 1.5 Rz/μm to about 25Rz/μm. As another example, the current collector can have a surfaceroughness from about 3 Rz/μm to about 25 Rz/μm.

Some electrodes can further comprise at least one second phase withinthe composite material. The at least one second phase can be configuredto bind electrochemically active particles of the electrochemicallyactive material together. The composite material can comprise particlesof a second metal, and the at least one second phase can comprise acompound of the second metal and the electrochemically active material.In some instances, the second metal can comprise copper, nickel, iron,titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof. The at least one second phase can comprise a metalsilicide. For example, the metal silicide can comprise copper silicide,nickel silicide, chromium silicide, aluminum silicide, titaniumsilicide, or a combination thereof.

In some electrodes, the composite material can further include fromgreater than 0% to about 95% by weight of one or more types of carbonphases. The electrochemically active material can include siliconparticles. The electrode can further comprise a metal silicide phaseconfigured to bind the silicon particles together and/or the siliconparticles to the one or more types of carbon phases. The metal silicidephase can include copper silicide, nickel silicide, chromium silicide,aluminum silicide, titanium silicide, or a combination thereof.

In some electrodes, the electrochemically active material can comprisesilicon particles from about 50% to about 99% by weight. For example,the electrochemically active material can comprise the silicon particlesfrom about 60% to about 99% by weight. As another example, theelectrochemically active material can comprise the silicon particlesfrom about 70% to about 99% by weight. In some instances, the electrodecan be a negative electrode.

Example electrochemical cells are also provided. The electrochemicalcell can include any of the electrodes described herein. Theelectrochemical cell can include a lithium ion battery, a magnesiumbattery, an aluminum battery, a sodium battery, or a combinationthereof.

Example methods of forming an electrode are provided. The method caninclude providing a current collector comprising one or more layers ofmetal, providing a precursor comprising from greater than 0% to about99% by weight of electrochemically active material, and heating theprecursor and the current collector to form a composite material and atleast one phase between the composite material and the currentcollector. The at least one phase can be configured to adhere thecomposite material to the current collector. The at least one phase cancomprise a compound of the metal and the electrochemically activematerial.

In some examples, the current collector can comprise copper, nickel,iron, titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof. The electrochemically active material can comprisesilicon, graphite, germanium, tin, silicon oxide (SiOx), aluminum, or acombination thereof. In some instances, the compound can comprise ametal silicide. For example, the metal silicide can include coppersilicide, nickel silicide, chromium silicide, aluminum silicide,titanium silicide, or a combination thereof.

In some instances, the current collector can have a surface roughnessfrom about 0.025 Rz/μm to about 25 Rz/μm. As an example, the surfaceroughness can be from about 1.5 Rz/μm to about 25 Rz/μm. As anotherexample, the surface roughness can be from about 3 Rz/μm to about 25Rz/μm.

In some methods, providing the precursor can include coating a mixtureon the current collector and drying the mixture. Heating the precursorand the current collector can include heating at a temperature fromabout 300° C. to about 900° C. For example, the temperature can be fromabout 650° C. to about 900° C. Heating the precursor and the currentcollector can include forming at least one second phase within thecomposite material.

In various methods, providing the precursor can comprise providing metalparticles of a second metal within the precursor. The metal particlescan include copper, nickel, iron, titanium, molybdenum, stainless steel,chromium, aluminum, or a combination thereof. The at least one secondphase can comprise a compound of the second metal and theelectrochemically active material. The compound can be configured tobind electrochemically active particles of the electrochemically activematerial together. The at least one second phase can comprise a metalsilicide. For example, the metal silicide can include copper silicide,nickel silicide, chromium silicide, aluminum silicide, titaniumsilicide, or a combination thereof.

In some instances, the composite material can comprise from greater than0% to about 95% by weight of one or more types of carbon phases. In someinstances, the electrochemically active material can comprise siliconparticles.

In some methods, heating the precursor and the current collector canfurther comprise forming a metal silicide phase configured to bind thesilicon particles together and/or the silicon particles to the one ormore types of carbon phases. The metal silicide phase can comprisecopper silicide, nickel silicide, chromium silicide, aluminum silicide,titanium silicide, or a combination thereof.

In some examples, the electrochemically active material can comprisesilicon particles from about 50% to about 99% by weight. For example,the electrochemically active material can comprise the silicon particlesfrom about 60% to about 99% by weight. As another example, theelectrochemically active material can comprise the silicon particlesfrom about 70% to about 99% by weight.

Example composite materials are also provided. The composite materialcan include from greater than 0% to about 99% by weight ofelectrochemically active material. The composite material can alsoinclude at least one phase between electrochemically active particles ofthe electrochemically active material. The at least one phase can beconfigured to bind the electrochemically active particles together. Theat least one phase can comprise a compound of metal and theelectrochemically active material.

In some examples, the electrochemically active material can comprisesilicon, graphite, germanium, tin, silicon oxide (SiOx), aluminum, or acombination thereof. The metal can comprise copper, nickel, iron,titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof. The at least one phase can comprise a metalsilicide phase. For example, the metal silicide phase can include coppersilicide, nickel silicide, chromium silicide, aluminum silicide,titanium silicide, or a combination thereof.

Some composite materials can further include from greater than 0% toabout 95% by weight of one or more types of carbon phases. Theelectrochemically active material can comprise silicon particles. The atleast one phase can include a metal silicide phase configured to bindthe silicon particles together and/or the silicon particles to the oneor more types of carbon phases. The metal silicide phase can includecopper silicide, nickel silicide, chromium silicide, aluminum silicide,titanium silicide, or a combination thereof.

In some instances, the electrochemically active material can includesilicon particles from about 50% to about 99% by weight. For example,the electrochemically active material can include the silicon particlesfrom about 60% to about 99% by weight. As another example, theelectrochemically active material can include the silicon particles fromabout 70% to about 99% by weight.

Example electrodes which comprise any of the composite materialsdescribed herein can also be provided. The electrode can further includea current collector. The electrode can be a negative electrode.

Example electrochemical cells which comprise any of the compositematerials described herein are also provided. The electrochemical cellcan include a lithium ion battery, a magnesium battery, an aluminumbattery, a sodium battery, or a combination thereof.

Example methods of forming composite material are also provided. Themethod can include providing a precursor comprising from greater than 0%to about 99% by weight of electrochemically active material, providingparticles of a metal within the precursor, and heating the precursorcomprising the electrochemically active material and the metal particlesto form at least one phase between electrochemically active particles ofthe electrochemically active material. The at least one phase can beconfigured to bind the electrochemically active particles together. Theat least one phase can comprise a compound of the metal and theelectrochemically active material.

In some methods, heating can comprise heating at a temperature fromabout 300° C. to about 900° C. For example, the temperature can be fromabout 650° C. to about 900° C. In some implementations, theelectrochemically active material can comprise silicon, graphite,germanium, tin, silicon oxide (SiOx), aluminum, or a combinationthereof. In some methods, providing metal particles can includeproviding copper, nickel, iron, titanium, molybdenum, stainless steel,chromium, aluminum, or a combination thereof. The at least one phase cancomprise metal silicide. For example, the metal silicide phase caninclude copper silicide, nickel silicide, chromium silicide, aluminumsilicide, titanium silicide, or a combination thereof.

Providing metal particles can include providing the metal particles inan amount from greater than 0% to about 30% by weight of the precursor.The precursor can include from greater than 0% to about 95% by weight ofone or more types of carbon phases. In some examples, theelectrochemically active material can comprise silicon particles. The atleast one phase can include a metal silicide phase configured to bindthe silicon particles together and/or the silicon particles to the oneor more types of carbon phases. In some examples, the metal silicidephase comprises copper silicide, nickel silicide, chromium silicide,aluminum silicide, titanium silicide, or a combination thereof.

In some instances, the precursor can comprise the silicon particles fromabout 50% to about 99% by weight. For example, the precursor can includethe silicon particles from about 60% to about 99% by weight. As anotherexample, the precursor can include the silicon particles from about 70%to about 99% by weight.

In some methods, providing the precursor can include providing theprecursor on a current collector. Heating the precursor can includeheating the precursor on the current collector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an example electrode in accordance withcertain embodiments described herein.

FIG. 2 illustrates an example method of forming an electrode.

FIG. 3 schematically illustrates an example composite material inaccordance with certain embodiments described herein.

FIG. 4 illustrates an example method of forming a composite material.

FIG. 5A schematically illustrates copper silicide particles at theinterface of a Si/C anode film and Cu foil. FIGS. 5B and 5C are ScanningElectron Microscopy (SEM) images of the interface between a Si/C anodefilm and Cu foil in accordance with certain embodiments describedherein.

FIG. 6 is an X-Ray Diffraction (XRD) pattern showing the presence of theintermetallic silicide phase Cu₃Si at the interface between a Si/C anodefilm and Cu foil in accordance with certain embodiments describedherein.

FIG. 7 is an XRD pattern showing presence of a copper silicide phaseafter treatment at 650° C.

FIG. 8 is an XRD pattern showing presence of a copper silicide phaseafter treatment at 900° C.

FIG. 9 is an XRD pattern showing presence of a copper silicide phaseafter treatment at 1175° C.

FIG. 10 is an XRD pattern showing presence of nickel silicide phaseafter treatment at 650° C.

FIG. 11 is an XRD pattern showing presence of nickel silicide phaseafter treatment at 900° C.

FIG. 12 is an XRD pattern showing presence of nickel silicide phaseafter treatment at 1175° C.

FIG. 13 shows the capacity retention of a cycled full cell having anexample anode as described herein.

FIG. 14A and FIG. 14B are SEM images of copper substrates with a surfaceroughness of 1.33 Rz/μm and 4.06 Rz/μm respectively.

FIG. 15 shows two copper substrates with different surface roughnessafter an adhesion test.

FIG. 16 shows the capacity retention of cycled full cells having exampleanodes as described herein.

DETAILED DESCRIPTION

This application describes certain embodiments of electrodes (e.g.,anodes and cathodes) and electrochemical cells. The electrodes andelectrochemical cells can include a composite material comprisingelectrochemically active material. In some embodiments, the compositematerial may include carbonized polymer and silicon material. Forexample, a mixture that includes a carbon precursor including siliconmaterial can be formed into a composite material. This mixture caninclude both carbon and silicon and thus can be referred to as acarbon-silicon composite material, a silicon-carbon composite material,a carbon composite material, or a silicon composite material.

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. Anodeelectrodes used in the rechargeable lithium-ion cells typically have aspecific capacity of approximately 200 milliamp hours per gram(including the metal foil current collector, conductive additives, andbinder material). Graphite, the active material used in most lithium ionbattery anodes, has a theoretical energy density of 372 milliamp hoursper gram (mAh/g). In comparison, silicon has a high theoretical capacityof 4200 mAh/g. Silicon, however, swells in excess of 300% uponlithiation. Because of this expansion, anodes including silicon mayexpand/contract and lose electrical contact to the rest of the anode.Accordingly batteries with silicon anodes exhibit more rapid capacityloss upon cycling than those batteries with graphite anodes. Therepeated expansion and contraction of silicon particles during chargeand discharge can lead to mechanical failure of the anode duringcycling, including disconnection between silicon and carbon, silicon andcurrent collectors, as well as the disconnection between siliconparticles. Therefore, a silicon anode should be designed to be able toexpand while maintaining good electrical contact with the rest of theelectrode.

Polymer binders have been used to provide adhesion between siliconparticles and metal current collectors (e.g., copper current collectors)and cohesion between the silicon particles. Polyimide polymers can beused as a binder in silicon-carbon anodes after high temperature curingabove 300° C. In some instances, heat treatment at a higher temperaturemay be desired to improve Coulombic efficiency by reducing the sidereactions. However, the high temperature curing may weaken adhesionbetween the anode film and the metal current collector, e.g., becausethe polymer can partially decompose upon the heat treatment.

Various embodiments described herein include introducing at least onephase to advantageously improve adhesion between an electrochemicallyactive composite material and a current collector. In addition, variousembodiments described herein can include at least one phase toadvantageously improve cohesion between electrochemically activeparticles and/or between electrochemically active particles and othermaterials in a composite material (e.g., between silicon particlesand/or between silicon particles and carbon phases).

Adhesion Between Composite Material and Current Collector

FIG. 1 schematically illustrates an example electrode in accordance withcertain embodiments described herein. The electrode 100 can be used as anegative electrode (e.g., an anode), a positive electrode (e.g., acathode), or both. Various embodiments of the electrode 100 can be usedin either secondary batteries (e.g., rechargeable) or primary batteries(e.g., non-rechargeable). The electrochemical cell can include a lithiumion battery, a magnesium battery, an aluminum battery, a sodium battery,or a combination thereof.

With continued reference to FIG. 1, the example electrode 100 caninclude a current collector 110, a composite material 120, and at leastone phase 130 configured to adhere (or enhance adhesion of or provideadditional adhesion of) the composite material 120 to the currentcollector 110. The current collector can include any current collectorknown in the art or yet to be developed. In various embodiments, thecurrent collector 110 can include one or more layers of metal. Forexample, the current collector 110 can include a foil or a clad foil. Asanother example, the current collector 110 can include one or more metalcoatings (e.g., a layer of metal disposed on the current collector).Example metals include, but are not limited to, copper, nickel, iron,titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof.

In various embodiments, the composite material 120 can be in electricalcommunication with the current collector 110. The composite material 120can include electrochemically active material. In some embodiments, thecomposite material 120 can include from greater than 0% to about 99% byweight of electrochemically active material. For example, the amount ofelectrochemically active material by weight of the composite materialcan include any weight percent within this range (e.g., about 10%, about15%, about 20%, about 25%, about 30%, about 35%, about 40%, about 45%,about 50%, about 55%, about 60%, about 65%, about 70%, about 75%, about80%, about 85%, about 90%, about 95%, about 96%, about 97%, about 98%,about 99%, etc.), or any range within this range such as any rangeformed by the example values (e.g., greater than about 0% to about 25%by weight, greater than about 0% to about 35% by weight, greater thanabout 0% to about 50% by weight, greater than about 0% to about 70% byweight, greater than about 0% to about 90% by weight, greater than about0% to about 95% by weight, from about 10% to about 35% by weight, fromabout 10% to about 50% by weight, from about 10% to about 90% by weight,from about 10% to about 95% by weight, from about 10% to about 99% byweight, from about 30% to about 85% by weight, from about 30% to about90% by weight, from about 30% to about 95% by weight, from about 30% toabout 99% by weight, from about 50% to about 85% by weight, from about50% to about 90% by weight, from about 50% to about 95% by weight, fromabout 50% to about 99% by weight, from about 60% to about 85% by weight,from about 60% to about 90% by weight, from about 60% to about 95% byweight, from about 60% to about 99% by weight, from about 70% to about85% by weight, from about 70% to about 90% by weight, from about 70% toabout 95% by weight, from about 70% to about 99% by weight, etc.).

The electrochemically active material can include any electrochemicallyactive material. For example, the electrochemically active material caninclude silicon, graphite, germanium, tin, silicon oxide (SiOx),aluminum, or a combination thereof. As described herein, variousembodiments can include a silicon-carbon (or carbon-silicon) compositematerial. U.S. patent application Ser. No. 13/008,800 (U.S. Pat. No.9,178,208), U.S. patent application Ser. No. 13/601,976 (U.S. PatentApplication Publication No. 2014/0170498), and U.S. patent applicationSer. No. 13/799,405 (U.S. Pat. No. 9,553,303), each of which isincorporated by reference herein, describe certain embodiments ofcarbon-silicon composite materials using carbonized polymer and siliconmaterial.

In various embodiments, at least one phase 130 can be between thecomposite material 120 and the current collector 110. FIG. 1schematically illustrates phase 130 with a cross-sectional shape similarto an oval. However, the shape, size, number, and/or distribution ofphase 130 are not particularly limited.

As described herein, phase 130 can act as a binding agent or bindingmaterial. For example, phase 130 can be configured to adhere (or enhanceadhesion or provide additional adhesion of) the composite material 120to the current collector 110. In some embodiments, other binding agentsor binding material (e.g., polymer) can be used to enhance the bonding.For example, in some instances, phase 130 can provide the main bindingmechanism and another binder may also be used. Phase 130 can include acompound of the metal of the current collector 110 and theelectrochemically active material of the composite material 120. Withoutbeing bound by theory, phase 130 can be formed (e.g., grown) via a heattreatment that may facilitate diffusion and/or reactions between themetal of the current collector 110 (or a metal disposed on the currentcollector 110) and the electrochemically active material of thecomposite material 120 at the interface, without adversely affecting thecurrent collector and/or the electrochemically active material (e.g.,without facilitating reactions that may destroy the current collectorand/or pulverize the electrochemically active material). For example, insome instances, the amount of reaction between the electrochemicallyactive material and the current collector 110 and/or the depth of thecurrent collector 110 that reacts with the electrochemically activematerial (e.g., the reaction depth) can be about 10% or less (e.g.,about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about 7%,about 7.5%, about 8%, about 9%, about 10%, etc.) or any range formed bysuch example values (e.g., from about 1% to about 5%, from about 1% toabout 7.5%, from about 1% to about 10%, from about 2% to about 5%, fromabout 2% to about 7.5%, from about 2% to about 10%, from about 3% toabout 5%, from about 3% to about 7.5%, from about 3% to about 10%, fromabout 5% to about 10%, from about 7.5% to about 10%, etc.) As anexample, a composite material 120 can be coated on a metal currentcollector 110, and heated in a range from about 300° C. to about 900° C.for a duration of about 24 hours to form phase 130 comprising a compoundof the metal of the current collector 110 and the electrochemicallyactive material of the composite material 120. For carbon-siliconcomposite materials (e.g., composite materials comprising silicon) on acopper or nickel foil, phase 130 comprising a metal silicide (e.g.,copper silicide or nickel silicide) can form. Other example metalsilicides can include chromium silicide, aluminum silicide, and/ortitanium silicide (e.g., with chromium, aluminum, and/or titaniumcurrent collectors).

In some embodiments, phase 130 can form when the electrochemicallyactive material and the metal of the current collector 110 are inphysical contact with each other. Surfaces with relatively high surfacearea can provide more contact points. Without being bound by theory, anincrease in the number of contact points between the electrochemicallyactive material and the current collector can promote reactions andfurther improve adhesion. In some embodiments, the current collector 110can have a surface roughness from about 0.025 Rz/μm to about 25 Rz/μm.For example, the current collector 110 can have a surface roughnesswithin this range (e.g., about 0.025 Rz/μm, about 1 Rz/μm, about 1.5Rz/μm, about 2 Rz/μm, about 2.5 Rz/μm, 3 Rz/μm, about 3.5 Rz/μm, about 4Rz/μm, about 4.5 Rz/μm, etc.), or any range within this range such asany range formed by the example values (e.g., greater than about 0.03Rz/μm to about 25 Rz/μm, greater than about 0.05 Rz/μm to about 25Rz/μm, greater than about 1 Rz/μm to about 25 Rz/μm, greater than about1.5 Rz/μm to about 25 Rz/μm, greater than about 2 Rz/μm to about 25Rz/μm, greater than about 2.5 Rz/μm to about 25 Rz/μm, greater thanabout 3 Rz/μm to about 25 Rz/μm, greater than about 3.5 Rz/μm to about25 Rz/μm, greater than about 4 Rz/μm to about 25 Rz/μm, etc.). In someembodiments, a current collector with a relatively high surface area(e.g., due to the surface roughness) can increase the number of contactpoints.

FIG. 2 illustrates an example method of forming an electrode (e.g., theelectrode 100 schematically illustrated in FIG. 1). The method 200 offorming can include providing a current collector (as shown in block210), providing a precursor comprising electrochemically active materialon the current collector (as shown in block 220), and heating theprecursor and the current collector to form a composite material and atleast one phase between the composite material and the current collector(as shown in block 230).

With reference to block 210, the provided current collector can includeany of the current collectors described herein. With reference to block210, the provided precursor can include from greater than 0% to about99% by weight of electrochemically active material. For example, theamount of electrochemically active material by weight of the precursorcan include any weight percent within this range (e.g., about 10%, about15%, about 20%, about 25%, about 30%, about 35%, about 40%, about 45%,about 50%, about 55%, about 60%, about 65%, about 70%, about 75%, about80%, about 85%, about 90%, about 95%, about 96%, about 97%, about 98%,about 99%, etc.), or any range within this range such as any rangeformed by the example values (e.g., greater than about 0% to about 25%by weight, greater than about 0% to about 35% by weight, greater thanabout 0% to about 50% by weight, greater than about 0% to about 90% byweight, greater than about 0% to about 95% by weight, from about 10% toabout 35% by weight, from about 10% to about 50% by weight, from about10% to about 90% by weight, from about 10% to about 95% by weight, fromabout 10% to about 99% by weight, from about 30% to about 85% by weight,from about 30% to about 90% by weight, from about 30% to about 95% byweight, from about 30% to about 99% by weight, from about 50% to about85% by weight, from about 50% to about 90% by weight, from about 50% toabout 95% by weight, from about 50% to about 99% by weight, from about60% to about 85% by weight, from about 60% to about 90% by weight, fromabout 60% to about 95% by weight, from about 60% to about 99% by weight,from about 70% to about 85% by weight, from about 70% to about 90% byweight, from about 70% to about 95% by weight, from about 70% to about99% by weight, etc.). The electrochemically active material can includeany electrochemically active material as described herein, including butnot limited to silicon, graphite, germanium, tin, silicon oxide (SiOx),aluminum, or a combination thereof.

The precursor can include any of the precursors for carbon-siliconcomposite materials as described in U.S. patent application Ser. No.13/008,800, U.S. patent application Ser. No. 13/601,976, and U.S. patentapplication Ser. No. 13/799,405, each of which is incorporated byreference herein. As an example, the electrochemically active materialcan include various types of silicon materials, including but notlimited to silicon powders, silicon fibers, porous silicon, ball-milledsilicon, etc. In some embodiments, a mixture of the precursor can becoated on the current collector and dried.

With reference to block 230, the method 200 can also include heating theprecursor and the current collector to form a composite material and atleast one phase between the composite material and the currentcollector. The mixture of the precursor on the current collector can beheated at a temperature from about 300° C. to about 900° C., from about400° C. to about 900° C., from about 500° C. to about 900° C., fromabout 600° C. to about 900° C., from about 650° C. to about 900° C.,etc. In some instances, a higher temperature may have a lower processingtime. In some instances, a lower temperature may have a higherprocessing time. The process to convert the precursor into a compositematerial and/or the formed composite material can be any of thosedescribed herein or in U.S. patent application Ser. No. 13/008,800, U.S.patent application Ser. No. 13/601,976, and U.S. patent application Ser.No. 13/799,405. For example, the precursor can be a mixture that ispyrolyzed to form a carbon-silicon composite material. The phase thatforms can include any phase described herein (e.g., a compound of themetal of the current collector and the electrochemically active materialof the composite material). As described herein, the phase can act as abinding material to adhere (or improve adhesion or provide additionaladhesion) between the composite material and the current collector.Compared to some processes described in U.S. patent application Ser. No.13/008,800, U.S. patent application Ser. No. 13/601,976, and U.S. patentapplication Ser. No. 13/799,405, some embodiments described herein canpyrolyze the composite material at a lower temperature to control thereactions between the current collector and the electrochemically activematerial of the composite material. In some instances, the amount ofreaction between the electrochemically active material and the currentcollector 110 and/or the depth of the current collector 110 that reactswith the electrochemically active material (e.g., the reaction depth)can be about 10% or less as described herein. In some instances, thecarbon may not be fully pyrolyzed, e.g., some hydrogen may be remaining.

Cohesion within the Composite Material

FIG. 3 schematically illustrates an example composite material inaccordance with certain embodiments described herein. As describedherein, the composite material 300 can include electrochemically activematerial 320. In some embodiments, the composite material 300 caninclude from greater than 0% to about 99% by weight of electrochemicallyactive material 320. For example, the amount of electrochemically activematerial 320 by weight of the composite material 300 can include anyweight percent within this range (e.g., about 10%, about 15%, about 20%,about 25%, about 30%, about 35%, about 40%, about 45%, about 50%, about55%, about 60%, about 65%, about 70%, about 75%, about 80%, about 85%,about 90%, about 95%, about 96%, about 97%, about 98%, about 99%, etc.),or any range within this range such as any range formed by the examplevalues (e.g., greater than about 0% to about 25% by weight, greater thanabout 0% to about 35% by weight, greater than about 0% to about 50% byweight, greater than about 0% to about 90% by weight, greater than about0% to about 95% by weight, from about 10% to about 35% by weight, fromabout 10% to about 50% by weight, from about 10% to about 90% by weight,from about 10% to about 95% by weight, from about 10% to about 99% byweight, from about 30% to about 85% by weight, from about 30% to about90% by weight, from about 30% to about 95% by weight, from about 30% toabout 99% by weight, from about 50% to about 85% by weight, from about50% to about 90% by weight, from about 50% to about 95% by weight, fromabout 50% to about 99% by weight, from about 60% to about 85% by weight,from about 60% to about 90% by weight, from about 60% to about 95% byweight, from about 60% to about 99% by weight, from about 70% to about85% by weight, from about 70% to about 90% by weight, from about 70% toabout 95% by weight, from about 70% to about 99% by weight, etc.). Theelectrochemically active material 320 can include any of theelectrochemically active materials 320 described herein, including butnot limited to silicon, graphite, germanium, tin, silicon oxide (SiOx),aluminum, or a combination thereof.

In various embodiments, at least one phase 330 can be betweenelectrochemically active particles of the electrochemically activematerial 320. In some embodiments, at least one phase 330 can be betweenthe electrochemically active particles of the electrochemically activematerial 320 and other materials 340 in the composite material 300. FIG.3 schematically illustrates phase 330 with a cross-sectional shapesimilar to an oval. However, the shape, size, number, and/ordistribution of phase 330 are not particularly limited.

As described herein, phase 330 can be configured to bind together (orimprove binding of or provide additional binding of) theelectrochemically active particles of the electrochemically activematerial 320. As also described herein, phase 330 can be configured tobind (or improve binding of or provide additional binding of) theelectrochemically active particles of the electrochemically activematerial 320 and other materials 340 in the composite material 300. Asan example, in carbon-silicon composite material (e.g., any of thecarbon-silicon composite materials described herein), phase 330 can beconfigured to bind or help bind silicon together and/or silicon andcarbon together.

In some embodiments, metal particles (e.g., powders) can be added to aprecursor comprising the electrochemically active material 320. When theprecursor is heated to form the composite material 300, at least onephase 330 can form between the materials in the composite material 300(e.g., between electrochemically active particles of theelectrochemically active material 320 and/or between theelectrochemically active particles of the electrochemically activematerial 320 and other materials in the composite material 300). Phase330 can include a compound of the added metal (e.g., from the metalparticles) and the electrochemically active material. The metals caninclude various metals including but not limited to copper, nickel,iron, titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof.

For carbon-silicon composite materials (e.g., composite materialscomprising silicon), phase 330 comprising a metal silicide can form.Example metal silicides can include copper silicide, nickel silicide,chromium silicide, aluminum silicide, titanium silicide, or acombination thereof (e.g., with copper, nickel, chromium, aluminum,and/or titanium particles).

In various embodiments, the composite material 300 can be used in anelectrode and/or electrochemical cell. For example, as described herein(e.g., with reference to FIG. 1), composite material can be inelectrical communication with a current collector 110. At least onephase 130 can be configured to adhere (or improve adhesion of or provideadditional adhesion of) the composite material 120 to the currentcollector 110. The electrode can be used as a negative or positiveelectrode in electrochemical cells. Various embodiments can be used insecondary or primary batteries. The electrochemical cell can include alithium ion battery, a magnesium battery, an aluminum battery, a sodiumbattery, or a combination thereof.

FIG. 4 illustrates an example method of forming a composite material(e.g., the composite material 300 schematically illustrated in FIG. 3).The method 400 of forming can include providing a precursor comprisingelectrochemically active material (as shown in block 410), providingparticles of metal within the precursor (as shown in block 420), andheating the precursor to form at least one phase between theelectrochemically active particles of the electrochemically activematerial (as shown in block 430). With reference to block 410, theprovided precursor can include any precursor described herein (e.g., theprovided precursor discussed with reference to FIG. 2) or any of theprecursors for carbon-silicon composite materials as described in U.S.patent application Ser. No. 13/008,800, U.S. patent application Ser. No.13/601,976, and U.S. patent application Ser. No. 13/799,405. Theelectrochemically active material can include any electrochemicallyactive material as described herein, including but not limited tosilicon, graphite, germanium, tin, silicon oxide (SiOx), aluminum, or acombination thereof.

With reference to block 420, the metal can include any of the metalsdescribed herein, including but not limited to copper, nickel, iron,titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof. In some embodiments, the precursor can include fromgreater than 0% to about 30% by weight of metal particles. For example,the amount of metal particles by weight of the precursor can include anyweight percent within this range (e.g., about 1%, about 2%, about 3%,about 4%, about 5%, about 7%, about 10%, about 15%, about 20%, about25%, about 30%, etc.), or any range within this range such as any rangeformed by the example values (e.g., greater than about 0% to about 5% byweight, greater than about 0% to about 10% by weight, greater than about0% to about 15% by weight, greater than about 0% to about 20% by weight,greater than about 0% to about 25% by weight, from about 1% to about 5%by weight, from about 1% to about 10%, from about 1% to about 15%, fromabout 1% to about 20%, from about 1% to about 25%, from about 1% toabout 30% by weight, from about 2% to about 5% by weight, from about 2%to about 10%, from about 2% to about 15%, from about 2% to about 20%,from about 2% to about 25%, from about 2% to about 30% by weight, fromabout 5% to about 10% by weight, from about 5% to about 15%, from about5% to about 20%, from about 5% to about 25%, from about 5% to about 30%by weight, from about 10% to about 20% by weight, from about 10% toabout 25% by weight, from about 20% to about 30% by weight, etc.).

With reference to block 430, heating the precursor can form at least onephase between electrochemically active particles of theelectrochemically active material to bind the electrochemically activeparticles together and/or between the electrochemically active particlesof the electrochemically active material and other material within thecomposite material to bind the electrochemically active particles andthe other material. In some embodiments, the precursor can be heated ata temperature from about 300° C. to about 900° C., from about 400° C. toabout 900° C., from about 500° C. to about 900° C., from about 600° C.to about 900° C., from about 650° C. to about 900° C., etc. In somemethods (e.g., without heating on a current collector), the precursorcan be heated at higher temperatures. For example, in some instances,the precursor can be heated from about 900° C. to about 1350° C. In somecases, heating at a higher temperature may utilize a lower processingtime; and/or heating at a lower temperature may utilize a higherprocessing time.

For carbon-silicon composite materials (e.g., composite materialscomprising silicon), a phase comprising a metal silicide can form.Example metal silicides can include copper silicide, nickel silicide,chromium silicide, aluminum silicide, titanium silicide, or acombination thereof.

In various embodiments, the method 400 of forming a composite materialshown in FIG. 4 can be part of a method of forming an electrode and/orelectrochemical cell. For example, the precursor with metal particlescan be provided on a current collector. The precursor and currentcollector can be heated to form a composite material comprising at leastone phase between the electrochemically active particles of theelectrochemically active material. As another example, as describedherein (e.g., with reference to FIG. 2), the precursor described withrespect to FIG. 4 can be provided on a current collector. The precursorand the current collector can be heated to form a composite material andat least one phase between the composite material and the currentcollector.

Composite Materials and Methods of Forming the Same

U.S. patent application Ser. No. 13/008,800, U.S. patent applicationSer. No. 13/601,976, and U.S. patent application Ser. No. 13/799,405,each of which is incorporated by reference herein, describe certainembodiments of carbon-silicon composite materials using carbonizedpolymer and silicon material. The carbonized polymer can act as anexpansion buffer for silicon particles during cycling so that a highcycle life can be achieved. In certain embodiments, the resultingelectrode can be an electrode that is comprised substantially of activematerial. For example, the carbonized polymer can form a substantiallycontinuous conductive carbon phase(s) in the entire electrode as opposedto particulate carbon suspended in a non-conductive binder in one classof conventional lithium-ion battery electrodes. Because the polymer canbe converted into an electrically conductive and electrochemicallyactive matrix, the resulting electrode can be conductive enough that ametal foil or mesh current collector may be omitted, minimized, orreduced in some embodiments. Accordingly, in U.S. patent applicationSer. No. 13/008,800, application Ser. No. 13/601,976, and U.S. patentapplication Ser. No. 13/799,405, certain embodiments of monolithic,self-supported electrodes are disclosed. The electrodes can have a highenergy density of between about 500 mAh/g to about 3500 mAh/g that canbe due to, for example, 1) the use of silicon, 2) elimination orsubstantial reduction of metal current collectors, and 3) beingcomprised entirely or substantially entirely of active material.

The carbon-silicon composite material can be formed by forming a mixtureincluding a precursor, and pyrolyzing the precursor to convert theprecursor to a carbon phase. In certain embodiments, the carbonprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamideimide, polyamic acid, polyimide, etc. Other precursorsinclude phenolic resins, epoxy resins, and other polymers. The mixturecan further include a solvent. For example, the solvent can beN-methyl-pyrollidone (NMP). Other possible solvents include acetone,diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate,ethyl carbonate, dimethoxyethane, etc. Examples of precursor and solventsolutions include PI-2611 (HD Microsystems), PI-5878G (HD Microsystems)and VTEC PI-1388 (RBI, Inc.). PI-2611 is comprised of >60%n-methyl-2-pyrollidone and 10-30%s-biphenyldianhydride/p-phenylenediamine. PI-5878G is comprised of >60%n-methylpyrrolidone, 10-30% polyamic acid of pyromelliticdianhydride/oxydianiline, 10-30% aromatic hydrocarbon (petroleumdistillate) including 5-10% 1,2,4-trimethylbenzene. In certainembodiments, the amount of precursor (e.g., solid polymer) in thesolvent is about 10 wt. % to about 30 wt. %.

The mixture can include silicon particles as described herein. Themixture may comprise about 5% to about 99% by weight of the precursor,and from greater than 0% to about 99% by weight of the silicon particles(for example, at least about 20% to about 95% by weight of the siliconparticles, at least about 20% to about 99% by weight of the siliconparticles, etc.). Additional materials can also be included in themixture. As an example, carbon particles including graphite activematerial, chopped or milled carbon fiber, carbon nanofibers, carbonnanotubes, and other conductive carbons can be added to the mixture.Conductive particles can also be added to the mixture. In addition, themixture can be mixed to homogenize the mixture.

In certain embodiments, the mixture is cast on a substrate. In someembodiments, casting includes using a gap extrusion, tape casting, or ablade casting technique. The blade casting technique can includeapplying a coating to the substrate by using a flat surface (e.g.,blade) which is controlled to be a certain distance above the substrate.A liquid or slurry can be applied to the substrate, and the blade can bepassed over the liquid to spread the liquid over the substrate. Thethickness of the coating can be controlled by the gap between the bladeand the substrate since the liquid passes through the gap. As the liquidpasses through the gap, excess liquid can also be scraped off. Forexample, the mixture can be cast on a substrate comprising a polymersheet, a polymer roll, and/or foils or rolls made of glass or metal. Themixture can then be dried to remove the solvent. For example, a polyamicacid and NMP solution can be dried at about 110° C. for about 2 hours toremove the NMP solution.

In various embodiments, the substrate is a metal current collector, andthe dried mixture remains on the current collector for furtherprocessing (e.g., in some embodiments providing adhesion between thecomposite material and the current collector and cohesion within thecomposite material). In some other embodiments, the dried mixture can beremoved from the substrate (e.g., in some embodiments providing cohesionwithin the composite material). For example, an aluminum substrate canbe etched away with HCl. Alternatively, the dried mixture can be removedfrom the substrate by peeling or otherwise mechanically removing thedried mixture from the substrate. In some embodiments, the substratecomprises polyethylene terephthalate (PET), including for exampleMylar®. In certain embodiments, the dried mixture is a film or sheet.

In some embodiments, the dried mixture (on a substrate or removed fromthe substrate) is optionally cured. In some embodiments, the driedmixture may be further dried. For example, the dried mixture can placedin a hot press (e.g., between graphite plates in an oven). A hot presscan be used to further dry and/or cure and to keep the dried mixtureflat. For example, the dried mixture from a polyamic acid and NMPsolution can be hot pressed at about 200° C. for about 8 to 16 hours.Alternatively, the entire process including casting and drying can bedone as a roll-to-roll process using standard film-handling equipment.The dried mixture can be rinsed to remove any solvents or etchants thatmay remain. For example, de-ionized (DI) water can be used to rinse thedried mixture.

In certain embodiments, tape casting techniques can be used for castingon a substrate. In some embodiments, the mixture can be coated on asubstrate by a slot die coating process (e.g., metering a constant orsubstantially constant weight and/or volume through a set orsubstantially set gap). In some other embodiments, there is no substratefor casting and the anode film does not need to be removed from anysubstrate. The dried mixture may be cut or mechanically sectioned intosmaller pieces.

The mixture further goes through pyrolysis to convert the polymerprecursor to carbon. In certain embodiments, the mixture is pyrolysed ina reducing atmosphere. For example, an inert atmosphere, a vacuum and/orflowing argon, nitrogen, or helium gas can be used. In some embodiments,the mixture is heated in a range from about 300° C. to about 900° C. Forexample, polyimide formed from polyamic acid can be carbonized at atemperature in a range from about 300° C. to about 900° C. (e.g., 600°C., 650° C., 700° C., etc.) for about one hour. In some embodiments(e.g., without a substrate/current collector), the mixture can be heatedin a range from about 900° C. to about 1350° C. (e.g., at about 1175°C.). In certain embodiments, the heat up rate and/or cool down rate ofthe mixture is about 10° C./min. A holder may be used to keep themixture in a particular geometry. The holder can be graphite, metal,etc. In certain embodiments, the mixture is held flat. After the mixtureis pyrolysed, tabs can be attached to the pyrolysed material to formelectrical contacts. For example, nickel, copper or alloys thereof canbe used for the tabs.

In certain embodiments, one or more of the methods described herein canbe carried out in a continuous process. In certain embodiments, casting,drying, possibly curing and pyrolysis can be performed in a continuousprocess. For example, the mixture can be coated onto a glass or metalcylinder. The mixture can be dried while rotating on the cylinder tocreate a film. The film can be transferred as a roll or peeled and fedinto another machine for further processing. Extrusion and other filmmanufacturing techniques known in industry could also be utilized priorto the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles, creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material.

Silicon particles can increase the specific lithium insertion capacityof the composite material. When silicon absorbs lithium ions, itexperiences a large volume increase on the order of 300+ volume percentwhich can cause electrode structural integrity issues. In addition tovolumetric expansion related problems, silicon is not inherentlyelectrically conductive, but becomes conductive when it is alloyed withlithium (e.g., lithiation). When silicon de-lithiates, the surface ofthe silicon losses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan reduce the chance that electrical contact to the surface of thesilicon is lost. For example, the composite material can include carbonssuch as graphite which contributes to the ability of the composite toabsorb expansion and which is also capable of intercalating lithium ionsadding to the storage capacity of the electrode (e.g., chemicallyactive). Therefore, the composite material may include one or more typesof carbon phases.

As described herein, in order to increase volumetric and gravimetricenergy density of lithium-ion batteries, silicon may be used as theactive material for the cathode or anode. Several types of siliconmaterials, e.g., silicon nanopowders, silicon nanofibers, poroussilicon, and ball-milled silicon, are viable candidates as activematerials for the negative or positive electrode. As described herein,the amount of silicon provided in the mixture or in the compositematerial can be within a range from greater than about 0% to about 99%by weight of the composite material (e.g., about 20% to about 95% byweight of the composite material, about 20% to about 99% by weight ofthe composite material, etc.).

In some embodiments, all, substantially all, or at least some of thesilicon particles may have a particle size (e.g., the diameter or thelargest dimension of the particle) less than about 50 μm, less thanabout 40 μm, less than about 30 μm, less than about 20 μm, less thanabout 10 μm, less than about 1 μm, between about 10 nm and about 50 μm,between about 10 nm and about 40 μm, between about 10 nm and about 30μm, between about 10 nm and about 20 μm, between about 0.1 μm and about20 μm, between about 0.5 μm and about 20 μm, between about 1 μm andabout 20 μm, between about 1 μm and about 15 μm, between about 1 μm andabout 10 μm, between about 10 nm and about 10 μm, between about 10 nmand about 1 μm, less than about 500 nm, less than about 100 nm, andabout 100 nm. For example, in some embodiments, the average particlesize (or the average diameter or the average largest dimension) or themedian particle size (or the median diameter or the median largestdimension) of the silicon particles can be less than about 50 μm, lessthan about 40 μm, less than about 30 μm, less than about 20 μm, lessthan about 10 μm, less than about 1 μm, between about 10 nm and about 50μm, between about 10 nm and about 40 μm, between about 10 nm and about30 μm, between about 10 nm and about 20 μm, between about 0.1 μm andabout 20 μm, between about 0.5 μm and about 20 μm, between about 1 μmand about 20 μm, between about 1 μm and about 15 μm, between about 1 μmand about 10 μm, between about 10 nm and about 10 μm, between about 10nm and about 1 μm, less than about 500 nm, less than about 100 nm, andabout 100 nm. In some embodiments, the silicon particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

In certain embodiments, the silicon particles are at least partiallycrystalline, substantially crystalline, and/or fully crystalline.Furthermore, the silicon particles may or may not be substantially puresilicon. For example, the silicon particles may be substantially siliconor may be a silicon alloy. In one embodiment, the silicon alloy includessilicon as the primary constituent along with one or more otherelements.

Advantageously, the silicon particles described herein can improveperformance of electro-chemically active materials such as improvingcapacity and/or cycling performance. Furthermore, electro-chemicallyactive materials having such silicon particles may not significantlydegrade as a result of lithiation of the silicon particles.

The amount of carbon obtained from the precursor can be greater than 0%to about 95% by weight such as about 1% to about 95% by weight, about 1%to about 90% by weight, 1% to about 80% by weight, about 1% to about 70%by weight, about 1% to about 60% by weight, about 1% to about 50% byweight, about 1% to about 40% by weight, about 1% to about 30% byweight, about 5% to about 95% by weight, about 5% to about 90% byweight, about 5% to about 80% by weight, about 5% to about 70% byweight, about 5% to about 60% by weight, about 5% to about 50% byweight, about 5% to about 40% by weight, about 5% to about 30% byweight, about 10% to about 95% by weight, about 10% to about 90% byweight, about 10% to about 80% by weight, about 10% to about 70% byweight, about 10% to about 60% by weight, about 10% to about 50% byweight, about 10% to about 40% by weight, about 10% to about 30% byweight, about 10% to about 25% by weight, etc. For example, the amountof carbon obtained from the precursor can be about 1%, about 5%, about10% by weight, about 15% by weight, about 20% by weight, about 25% byweight, etc. from the precursor.

The carbon from the precursor can be hard carbon. Hard carbon can be acarbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. Hard carbon may be selected since soft carbonprecursors may flow and soft carbons and graphite are mechanicallyweaker than hard carbons. Other possible hard carbon precursors caninclude phenolic resins, epoxy resins, and other polymers that have avery high melting point or are crosslinked. The amount of hard carbon inthe composite material can be any of the ranges described herein withrespect to the amount of carbon obtained from the precursor. Forexample, in some embodiments, the amount of hard carbon in the compositematerial can have a value within a range of greater than 0% to about 95%by weight such as about 1% to about 95% by weight, about 1% to about 90%by weight, about 1% to about 80% by weight, about 1% to about 70% byweight, about 1% to about 60% by weight, about 1% to about 50% byweight, about 1% to about 40% by weight, about 1% to about 30% byweight, about 5% to about 95% by weight, about 5% to about 90% byweight, about 5% to about 80% by weight, about 5% to about 70% byweight, about 5% to about 60% by weight, about 5% to about 50% byweight, about 5% to about 40% by weight, about 5% to about 30% byweight, about 10% to about 95% by weight, about 10% to about 90% byweight, about 10% to about 80% by weight, about 10% to about 70% byweight, about 10% to about 60% by weight, about 10% to about 50% byweight, about 10% to about 40% by weight, about 10% to about 30% byweight, about 10% to about 25% by weight. In some embodiments, theamount of hard carbon in the composite material can be about 1% byweight, about 5% by weight, about 10% by weight, about 20% by weight,about 30% by weight, about 40% by weight, about 50% by weight, or morethan about 50% by weight. In certain embodiments, the hard carbon phaseis substantially amorphous. In other embodiments, the hard carbon phaseis substantially crystalline. In further embodiments, the hard carbonphase includes amorphous and crystalline carbon. The hard carbon phasecan be a matrix phase in the composite material. The hard carbon canalso be embedded in the pores of the additives including silicon. Thehard carbon may react with some of the additives to create somematerials at interfaces. For example, there may be a silicon carbidelayer between silicon particles and the hard carbon.

In some embodiments, graphite is one of the types of carbon phases fromthe precursor. In certain embodiments, graphite particles are added tothe mixture. Advantageously, graphite can be an electrochemically activematerial in the battery as well as an elastic deformable material thatcan respond to volume change of the silicon particles. Graphite is thepreferred active anode material for certain classes of lithium-ionbatteries currently on the market because it has a low irreversiblecapacity. Additionally, graphite is softer than hard carbon and canbetter absorb the volume expansion of silicon additives. In certainembodiments, all, substantially all, or at least some of the graphiteparticles may have a particle size (e.g., a diameter or a largestdimension) between about 0.5 microns and about 20 microns. In someembodiments, an average particle size (e.g., an average diameter or anaverage largest dimension) or median particle size (e.g., a mediandiameter or a median largest dimension) of the graphite particles isbetween about 0.5 microns and about 20 microns. In some embodiments, thegraphite particles may have a distribution of particle sizes. Forexample, at least about 95%, at least about 90%, at least about 85%, atleast about 80%, at least about 70%, or at least about 60% of theparticles may have the particle size described herein. In certainembodiments, the composite material can include graphite particles in anamount greater than 0% and less than about 80% by weight, including from40% to about 75% by weight, from about 5% to about 30% by weight, from5% to about 25% by weight, from 5% to about 20% by weight, or from 5% toabout 15% by weight.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particles canenable both a more electronically conductive composite as well as a moremechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, all, substantially all, or at least some of theconductive particles can have a particle size (e.g., the diameter or thelargest dimension) between about 10 nanometers and about 7 micrometers.In some embodiments, an average particle size (e.g., an average diameteror an average largest dimension) or a median particle size (e.g., amedian diameter or a median largest dimension) of the conductiveparticles is between about 10 nm and about 7 micrometers. In someembodiments, the conductive particles may have a distribution ofparticle sizes. For example, at least about 95%, at least about 90%, atleast about 85%, at least about 80%, at least about 70%, or at leastabout 60% of the particles may have the particle size described herein.

In certain embodiments, the mixture includes conductive particles in anamount greater than zero and up to about 80% by weight. In someembodiments, the composite material includes about 45% to about 80% byweight of conductive particles. The conductive particles can beconductive carbon including carbon blacks, carbon fibers, carbonnanofibers, carbon nanotubes, etc. Many carbons that are considered asconductive additives that are not electrochemically active become activeonce pyrolysed in a polymer matrix. Alternatively, the conductiveparticles can be metals or alloys including copper, nickel, or stainlesssteel. In some instances, the mixture and/or composite material caninclude greater than 0% to about 30% by weight of metal particles.

After the precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure (e.g., if removed from thesubstrate). The carbonized precursor results in an electrochemicallyactive structure that holds the composite material together. Forexample, the carbonized precursor can be a substantially continuousphase. Advantageously, the carbonized precursor can be a structuralmaterial as well as an electro-chemically active and electricallyconductive material. In certain embodiments, material particles added tothe mixture are homogenously or substantially homogeneously distributedthroughout the composite material to form a homogeneous or substantiallyhomogeneous composite.

In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder. Thecomposite material may also include porosity. In some embodiments, thecomposite material (or the film) can include porosity of about 1% toabout 70% or about 5% to about 50% by volume porosity. For example, theporosity can be about 5% to about 40% by volume porosity.

A current collector may be preferred in some applications, for example,where current above a certain threshold or additional mechanical supportmay be desired. In some such embodiments, the precursor described hereincan be pyrolyzed on the current collector as described herein. U.S.patent application Ser. No. 13/333,864, filed Dec. 21, 2011, andpublished on Jun. 19, 2014 as U.S. Patent Application Publication No.2014/0170482, entitled “Electrodes, Electrochemical Cells, and Methodsof Forming Electrodes and Electrochemical Cells,” and U.S. patentapplication Ser. No. 13/796,922, filed Mar. 12, 2013, and published onJun. 19, 2014 as U.S. Patent Application Publication No. 2014/0170475,entitled “Electrodes, Electrochemical Cells, and Methods of FormingElectrodes and Electrochemical Cells,” each of which is incorporated byreference herein, describe certain other embodiments of electrodesincluding a current collector, electrochemical cells comprising suchelectrodes, and methods of forming such electrodes and electrochemicalcells.

In some embodiments, the full capacity of the composite material of theelectrodes described herein may not be utilized during use of thebattery to improve life of the battery (e.g., number charge anddischarge cycles before the battery fails or the performance of thebattery decreases below a usability level). For example, a compositematerial with about 70% by weight of silicon particles, about 20% byweight of carbon from a precursor, and about 10% by weight of graphitemay have a maximum gravimetric capacity of about 3000 mAh/g, while thecomposite material may only be used up to an gravimetric capacity ofabout 550 to about 1500 mAh/g. Although, the maximum gravimetriccapacity of the composite material may not be utilized, using thecomposite material at a lower capacity can still achieve a highercapacity than certain lithium ion batteries. In certain embodiments, thecomposite material is used or only used at an gravimetric capacity belowabout 70% of the composite material's maximum gravimetric capacity. Forexample, the composite material is not used at an gravimetric capacityabove about 70% of the composite material's maximum gravimetriccapacity. In further embodiments, the composite material is used or onlyused at an gravimetric capacity below about 50% of the compositematerial's maximum gravimetric capacity or below about 30% of thecomposite material's maximum gravimetric capacity.

Examples

The following examples are provided to demonstrate the benefits of someembodiments of electrochemically active composite materials, electrodes,electrochemical cells, and methods of forming the same. These examplesare discussed for illustrative purposes and should not be construed tolimit the scope of the disclosed embodiments.

Example electrodes were prepared showing the growth of metal compoundparticles at the interface of an electrochemically active material and ametal substrate (e.g., to provide additional adhesion between the activematerial and the metal substrate). In one example (Example 1), anodeslurry was mixed with the formulation shown in Table 1, and was cast oncopper metal foil. The as-coated and dried anode was calendered toachieve a density of 1.1-1.3 g/cm³, followed by vacuum drying at 120° C.for 16 hours. After that, the anode was annealed/pyrolyzed at 750° C.for 2 hours under argon (Ar) atmosphere.

TABLE I Component Percentage (%) Silicon 22.64 NMP 17.86 Graphite 1.37Polyimide Resin 58.09 Surfactant 0.04

FIG. 5A schematically illustrates metal silicide particles (e.g.,Cu_(x)Si) at the interface of the silicon-carbon composite material(e.g., Si/C anode film) and the copper metal substrate (e.g., Cu foil).Without being bound by theory, it is believed that the Cu_(x)Si can workas a binding material formed between the Si/C anode film and Cu foil toprovide additional adhesion between the two materials. To detect theformation of a Cu_(x)Si phase at the interface, the anode was defoliatedfrom the Cu substrate with a strong adhesive tape to expose theinterface between the Cu foil and the Si/C anode film. FIGS. 5B and 5Care Scanning Electron Microscopy (SEM) images of the interface betweenthe Cu foil and the Si/C anode film. In FIG. 5B, a Cu_(x)Si particle iscomparatively brighter on the surface of the darker Si/C film. In FIG.5C, a low-magnification image shows the distribution of the Cu_(x)Siparticles on the Si/C anode film (e.g., on the side facing the Cu foilduring heat treatment). Without being bound by theory, the Si/C anodefilm surface where the highest points of the anode film would contactthe Cu foil have reacted and converted to a copper silicide indicatingthat the Cu_(x)Si particles are binding the active Si/C anode film andthe Cu foil. An X-Ray Diffraction (XRD) analysis was performed toidentify the phases of copper silicides on the exfoliated Si/C anode.FIG. 6 is an XRD pattern showing the presence of the intermetallicsilicide phase Cu₃Si at the interface between the Si/C anode film and Cufoil. The copper silicide phase was present along with silicon,graphite, and copper phases.

Example composite materials were also prepared to form a metal silicidephase between electrochemically active particles within an anode film.Without being bound by theory, it is believed that a metal silicidephase can provide additional cohesion between the active particles. Insome examples, to form a metal silicide phase between Si particleswithin an anode film, copper (Cu) and nickel (Ni) powders were addedinto the anode slurry as metal sources. To detect the formation of ametal silicide phase without complication from the current collector,the slurry was cast onto a polyethylene (PET) film instead of a metalfoil.

The anode slurry was made in a Centrifugal Planetary Mixer by addingvacuum dried Cu and Ni metal particles with sizes in the nanometer ormicrometer range (e.g., <100 μm) into a premixed and degassed slurrycontaining silicon, resin and NMP solvent. The composition of theformulation was roughly about 2% metal particles, 22-24% Si, 56%polyamic acid resin and 20% NMP. The slurry was hand-coated onto a PETfilm and dried in an oven at 80-160° C. for a few minutes. The coatedfilm was peeled off from the PET substrate and cured in a vacuum oven at200° C. overnight. The cured material was pyrolyzed in a furnace underinert atmosphere under different final temperatures and dwell times.

FIGS. 7-12 are XRD patterns showing the presence of metal silicides inSi/C anode films at different heat treatment temperatures. FIGS. 7-9show the formation of a copper silicide phase at a heat treatment of650° C., 900° C., and 1175° C. respectively for a duration of 1-2 hours.FIGS. 10-12 show the formation of a nickel silicide phase at a heattreatment of 650° C., 900° C., and 1175° C. respectively for a durationof 1-2 hours.

Example electrodes were prepared to test electrochemical performance. Inone example, an anode was prepared using similar process conditions asExample 1. The slurry was coated in a continuous roll process onto astainless steel core. Since Cu_(x)Si can be formed at 650° C., the rollwas annealed at 700° C. The full pouch stacked cell was prepared with aNCM622 based cathode and electrolyte of FEC:EMC=30:70 wt % at 1.0M ofLiPF₆. The cathode had NCM622 as an active material at 92 wt %, carbonadditive and PVDF as a binder, and a 15 micron thick Al foil. Thecathode had a 3.0 g/cm³ electrode density. FIG. 13 shows the capacityretention of the full cell cycled at Constant Current Constant Voltage(CCCV) at 4.2V and 0.05 C top off and discharge to 3.3V at 2.3 mA/cm².FIG. 13 shows stable cycling of the example full cell. Without beingbound by theory, it is believed that stable cycling is attributed to theformation of copper silicide acting as a binding agent to maintainelectrical contact between the Si/C composite anode film and Cu foil.

FIG. 14A is an SEM image of the Cu current collector used in Example 1.The copper current collector had a surface roughness of 1.33 Rz/μm. Withreference to FIG. 5C, although Cu₃Si formed on the Cu foil, in someembodiments, a higher amount of Cu₃Si phase may be desired. FIG. 14B isan SEM image of a Cu current collector having a surface roughness of4.06 Rz/μm. The slurry composition from Example 1 was coated on therougher Cu substrate followed by calendaring to have an electrodedensity at 1.3 g/cm³. After pyrolysis, an adhesion test was performed.FIG. 15 shows the results of the adhesion test for the current collector(Example A) having a 1.33 Rz/μm surface roughness and the currentcollector (Example B) having a 4.06 Rz/μm surface roughness. More anodematerial remained on the current collector with a higher surface area(e.g., due to a rougher current collector) indicating stronger adhesion(stronger interaction) between anode and Cu foil.

An example full pouch cell was prepared with a similar NCM622 basedcathode and electrolyte as used in the example cell cycled in FIG. 13,but with an anode having a current collector with a higher roughness.FIG. 16 shows the capacity retention of the example full cells cycled atConstant Current Constant Voltage (CCCV) at 4.2V and 0.05 C top off anddischarge to 3.3V at 2.3 mA/cm². FIG. 16 shows that the cycle retentionenhanced with the current collector having a rougher surface. Withoutbeing bound by theory, it is believed that the enhanced performance canbe attributed to more reactions between the Si and Cu.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of forming an electrode, the methodcomprising: providing a current collector comprising one or more layersof metal; providing a precursor comprising from greater than 0% to about99% by weight of electrochemically active material; and heating theprecursor and the current collector to form a composite material and atleast one phase between the composite material and the currentcollector; wherein the at least one phase is configured to adhere thecomposite material to the current collector, and wherein the at leastone phase comprises a compound of the metal and the electrochemicallyactive material, and wherein said at least one phase comprises localizedphases comprising particles of said compound surrounded by the compositematerial, the localized phases scattered across a plane between thecurrent collector and the composite material to a depth in the compositematerial of 10% or less of composite material thickness.
 2. The methodof claim 1, wherein the current collector comprises copper, nickel,iron, titanium, molybdenum, stainless steel, chromium, aluminum, or acombination thereof.
 3. The method of claim 1, wherein theelectrochemically active material comprises silicon, graphite,germanium, tin, silicon oxide (SiOx), aluminum, or a combinationthereof.
 4. The method of claim 1, wherein the compound comprises ametal silicide.
 5. The method of claim 4, wherein the metal silicidecomprises copper silicide, nickel silicide, chromium silicide, aluminumsilicide, titanium silicide, or a combination thereof.
 6. The method ofclaim 1, wherein the current collector has a surface roughness fromabout 0.025 Rz/μm to about 25 Rz/μm.
 7. The method of claim 6, whereinthe surface roughness is from about 1.5 Rz/μm to about 25 Rz/μm.
 8. Themethod of claim 7, wherein the surface roughness is from about 3 Rz/μmto about 25 Rz/μm.
 9. The method of claim 1, wherein providing theprecursor comprises coating a mixture on the current collector anddrying the mixture.
 10. The method of claim 1, wherein heating theprecursor and the current collector comprises heating at a temperaturefrom about 300° C. to about 900° C.
 11. The method of claim 10, whereinthe temperature is from about 650° C. to about 900° C.
 12. The method ofclaim 1, wherein heating the precursor and the current collectorcomprises forming at least one second phase within the compositematerial.
 13. The method of claim 12, wherein providing the precursorcomprises providing metal particles of a second metal within theprecursor.
 14. The method of claim 13, wherein the metal particlescomprise copper, nickel, iron, titanium, molybdenum, stainless steel,chromium, aluminum, or a combination thereof.
 15. The method of claim14, wherein the at least one second phase comprises a compound of thesecond metal and the electrochemically active material.
 16. The methodof claim 14, wherein the compound is configured to bindelectrochemically active particles of the electrochemically activematerial together.
 17. The method of claim 14, wherein the at least onesecond phase comprises a metal silicide.
 18. The method of claim 17,wherein the metal silicide comprises copper silicide, nickel silicide,chromium silicide, aluminum silicide, titanium silicide, or acombination thereof.
 19. The method of claim 1, wherein the compositematerial comprises from greater than 0% to about 95% by weight of one ormore types of carbon phases.
 20. The method of claim 19, wherein theelectrochemically active material comprises silicon particles.
 21. Themethod of claim 20, wherein heating the precursor and the currentcollector further comprises forming a metal silicide phase configured tobind the silicon particles together and/or the silicon particles to theone or more types of carbon phases.
 22. The method of claim 21, whereinthe metal silicide phase comprises copper silicide, nickel silicide,chromium silicide, aluminum silicide, titanium silicide, or acombination thereof.
 23. The method of claim 1, wherein theelectrochemically active material comprises silicon particles from about50% to about 99% by weight.
 24. The method of claim 23, wherein theelectrochemically active material comprises the silicon particles fromabout 60% to about 99% by weight.
 25. The method of claim 24, whereinthe electrochemically active material comprises the silicon particlesfrom about 70% to about 99% by weight.